\[\boxed{\text{П.47.}}\]
\[YX = 3\ см;\ \ YB = 11\ см;\ \ \]
\[AX = 7\ см.\ \]
\[XB = YB - YX = 11 - 3 = 8\ см.\ \]
\[AY = AX - YX = 7 - 3 = 4\ см.\ \]
\[AB = AY + YX + XB =\]
\[= 4 + 3 + 8 = 15\ см.\]
\[Ответ:AB = 15\ см.\ \ \ \ \]